In three weeks' time, new, shorter federal stopping distance limits go into effect for three-axle tractors, most of them over-the-road power units that pull semitrailers.
The new limits cut 30% off the current 355-foot distance from 60 mph, so the tractors have to stop in no more than 250 feet.

There'll be no new requirements for trailers and there aren't now, because they vary so much in weight that the feds long ago decided not to directly regulate them. Trailers tag along and with their brakes contribute to combination performance that meets the limits, experts say.

Changes to Federal Motor Vehicle Safety Standard 121, the air-brake regulations, take effect on August 1. They apply to tractors with a gross vehicle weight rating of up to 59,600 pounds. Heavier three- and four-axle tractors and those with two axles must meet the new limits two years later, on August 1, 2013.

The 60-mph speed has long been used to simulate highway cruising, and the distances mimic a panic stop from that speed. Experts say that with bigger drum brakes scheduled for tractors built starting August 1, far less than maximum air pressure must be used to meet the limits.

Tractors prove their performance on test tracks while pulling short trailers loaded to put the required 59,600 pounds on a tractor's three axles. The test trailer's brakes are disconnected so only the tractor's six sets of brakes (two at on each of its three axles) are applied.

In theory and actuality, a qualified tractor and trailer grossing 80,000 pounds, the federal limit on most Interstate highways, will also stop within today's 355 feet and next month's 250 feet, and probably less. That's because the combination has four more brakes (two on each axle of the trailer's tandem), but carries only another 20,400 pounds of weight.

Put another way, a trailer's brakes add roughly 36% more braking with only 27% more gross weight. That's a lot of braking for not many more pounds.

A similar situation will occur if Congress ever approves the concept of a 97,000-pound gross combination weight for a six-axle rig. The two extra brakes on the trailer's third axle will more than handle the extra 17,000 pounds of extra gross weight, experts say. So stopping performance and safety will be maintained, even if anti-big-and-heavy truck forces claim otherwise.